Currently, in the field of non-aqueous electrolyte secondary batteries, studies have been actively conducted on lithium ion secondary batteries having high voltages and high energy densities. The positive electrode active material of a lithium ion secondary battery is typically a lithium-containing transition metal oxide, such as LiCoO2, and the negative electrode active material is typically a carbon material. A secondary battery of this kind includes an electrolyte composed of a non-aqueous solvent and a solute dissolved therein. Exemplary non-aqueous solvents include cyclic carbonic acid ester, chain carbonic acid ester, and cyclic carboxylic acid ester. Exemplary solutes are lithium salts, such as lithium hexafluorophosphate (LiPF6) and lithium tetrafluoroborate (LiBF4).
With the aim of improving the characteristics of non-aqueous electrolyte secondary batteries, attempts have been made to add various additives to the positive electrode, negative electrode, and/or electrolyte. For example, in order to improve charge/discharge cycle characteristics or improve charge/discharge characteristics at low temperatures, there have been proposals to add vinylene carbonate or vinyl ethylene carbonate to an electrolyte (see Japanese Laid-Open Patent Publication No. 2003-151621, No. 2003-31259, and No. 2003-249262). Vinylene carbonate or vinyl ethylene carbonate is decomposed on the negative electrode to form a protective film, thereby suppressing the side reaction between the electrolyte and the negative electrode active material.
Even if vinylene carbonate or vinyl ethylene carbonate is contained in the electrolyte, the side reaction between the electrolyte and the positive electrode active material may violently occur particularly in a high-temperature environment, resulting in dramatic degradation in cycle characteristics. Particularly, when a highly reactive positive electrode active material as proposed by Japanese Laid-Open Patent Publication No. 2003-249262 is used, the cycle characteristics in a high-temperature environment tend to become insufficient. It is therefore an object of the present invention to provide a non-aqueous electrolyte secondary battery that exhibits good charge/discharge cycle characteristics particularly in a high-temperature environment.